Quality control plasma monitor for laser shock processing

ABSTRACT

A method and apparatus for quality control of laser shock processing. The method includes measuring emissions and characteristics of a workpiece when subjected to a pulse of coherent energy from a laser. These empirically measured emissions and characteristics of the workpiece are correlated to theoretical shock pressure, residual stress profile, or fatigue life of the workpiece. The apparatus may include a radiometer or acoustic detection device for measuring these characteristics.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a system for monitoring aworkpiece during laser shock processing, and in particular, a system formonitoring the pressure pulse applied to a workpiece during laser shockprocessing.

[0003] 2. Description of the Related Art

[0004] Laser shock processing involves a pulse of coherent radiation toa piece of solid material (workpiece) to produce shockwaves therein. Theproduced shockwave cold works the solid material to impart compressiveresidual stresses within the solid material. These compressive residualstresses improve the fatigue properties of the solid material.

[0005] Current laser shock processing utilizes two overlays: atransparent overlay (usually water), and an opaque layer (usually an oilbased paint or black plastic tape). During processing, a laser beam isdirected to pass through the transparent overlay and is absorbed by theopaque layer, causing a rapid vaporization of the opaque layer (plasmacreation) and generation of a high-amplitude shockwave. The shockwavecold works the surface of the part and creates deep compressive residualstresses which provide an increase in fatigue properties of theworkpiece. A workpiece is typically processed by employing a matrix ofoverlapping spots that cover the fatigue-critical zone of the part.

[0006] Currently, there is no known real-time method for measuring theshock pressure applied to a workpiece during laser shock peening. Whilecommercial pressure gauges, such as special quartz gauges or PVDF gaugesare available to make pressure measurements, these gauges must be usedoffline (not in real time on a workpiece). Furthermore, these gauges aresingle-use devices.

[0007] A quartz gauge is based on the piezoelectric behavior of quartzcrystals. When a pressure is applied to one surface of a quartz crystal,an electric current proportional to the stress difference between thissurface and the opposite surface is produced between electrodes attachedto these surfaces. The current then passes through a resistor and thevoltage measured across the resistor is proportional to the differencein the stress between the opposite surfaces. In the “thick gauge” mode,most or all of the shockwave passes into the thickness of the gaugebefore it reaches the opposite surface of the crystal. This enables oneto measure the entire shockwave profile directly.

[0008] One problem with current laser shock processing systems is thatthere is no real-time method or apparatus for measuring shock pressureor plasma characteristics during laser shock peening or correlating themto the imparted deep compressive residual stresses in a workpiece.Previous methods of measuring a pressure pulse applied to a workpieceincluded use of a quartz gauge. The disadvantage of using a quartz gaugeis that a quartz gauge is a single use instrument. In addition, the useof a quartz gauge does not permit real-time measuring of a pressurepulse while processing a workpiece. The use of a quartz gauge is limitedto measuring the pressure pulse applied to a workpiece either before orafter laser shock processing (i.e., not real time).

[0009] Another problem in the art is that there is no method forcorrelating plasma characteristics to a pressure pulse applied to aworkpiece.

[0010] Another problem in the art is that there is no known method orapparatus for real-time determination of imparted compressive residualstresses in a workpiece during laser shock peening. Currently, themethod of evaluating an imparted residual stress profile is to measurethe residual stresses using x-ray diffraction techniques. In order touse x-ray diffraction, a workpiece is normally removed from the lasershock processing station and placed in an x-ray machine, wherein anx-ray beam is directed to the workpiece surface to measure the residualstresses at that surface. In order to get an in-depth profile, asequence of thin layers is removed from the surface by electropolishing,then the surface residual stresses are measured between eachelectropolishing step. If only the residual stress at the originalsurface of the workpiece is measured, the measurements also includes theunknown effects of previous surface finishing processes. These usuallyvary from part to part and could be differentiated from the stressesimparted by laser shock peening. The technique of x-ray diffraction forin-depth profiles is a destructive method for evaluating compressiveresidual stresses imparted in a workpiece by laser shock peening. Theuse of x-ray diffraction is limited to post-laser shock peeninganalysis. Therefore, x-ray diffraction cannot be used as a real timemethod for determining imparted compressive residual stresses.

SUMMARY OF THE INVENTION

[0011] The present invention is a method and apparatus for monitoringlaser shock processing of a workpiece. The method and apparatus includesdetecting spectral and acoustic energy emitted from a workpiece or theenergy absorbing layer applied to the workpiece. The acoustic andspectral emissions may be correlated to the pressure pulse applied to aworkpiece, the residual stress profile produced in the workpiece, andthe fatigue life of the workpiece.

[0012] The invention, in one form thereof, is an apparatus formonitoring laser shock processing of a workpiece. The apparatus includesa material applicator for applying an energy absorbing material to theworkpiece. A transparent overlay applicator applies a transparentoverlay onto the workpiece over the energy absorbing layer. A laser isoperatively associated with the energy absorbing layer and there is atleast one radiometer. In one particular further embodiment, the energyabsorbing layer may contain a dopant.

[0013] The invention, in another form thereof, is a method for real timemonitoring laser shock processing of a workpiece. The method includesapplying an opaque overlay to the workpiece. A beam of coherent energyis directed to the workpiece to vaporize a portion of the opaque overlayand to create a plasma which emits energy therefrom. A portion of theenergy emitted from the plasma is monitored. In a further embodiment,spectral emissions are detected from the emitted energy. In an alternateembodiment, acoustic emissions are detected within the emitted energy.

[0014] The invention, in yet another form thereof, is a method for realtime monitoring the laser shock peening of a workpiece. The methodincludes applying a transparent overlay to a workpiece and directing abeam of coherent energy to the workpiece through the transparent overlayand to create a plasma which emits energy therefrom. A portion of theenergy emitted from the plasma is monitored. In a further embodiment,spectral emissions are detected from the plasma emitted energy. In analternate embodiment, acoustic emissions are detected from the emittedenergy. In alternate further embodiments, a feature of the spectralemissions or acoustic emissions are correlated to the shock pressureapplied to the workpiece.

[0015] The invention, in another form thereof, is a method for real timemonitoring the laser shock peening of a workpiece. The method includesapplying an opaque overlay to a workpiece and directing a beam ofcoherent energy to a workpiece to vaporize a portion of the opaqueoverlay and create a plasma thereon. The temperature of the plasma ismonitored. In a further embodiment, the plasma temperature is correlatedto the residual stress profile left in the workpiece.

[0016] The invention, in yet another form thereof, is a method for realtime monitoring the laser shock peening of a workpiece. The methodincludes applying a transparent overlay to a workpiece. A beam ofcoherent energy is directed to a workpiece through the transparentoverlay and creates a plasma thereon. The temperature of the plasma ismonitored. In further alternate embodiments, the plasma temperature iscorrelated to the shock pressure of the workpiece, the residual stressprofile left in the workpiece, and the fatigue life of the workpiece.

[0017] One advantage of the present invention is the ability tocorrelate characteristics and emissions from a workpiece and overlayduring laser shock peening to a shock pressure, compressive residualstress profile, or fatigue life of a workpiece. Characteristics orfeatures, such as acoustic emissions and spectral emissions arecorrelated to a shock pressure, compressive residual stress, or fatiguelife of a workpiece. As a result, monitoring such characteristics allowsone to predict the shock pressure and compressive residual stresses orfatigue life of a workpiece in real time.

[0018] Another advantage of the present invention is a non-destructivetechnique to predict the compressive residual stresses imparted in aworkpiece during laser shock processing. Through the correlation ofacoustic and spectral emissions from a workpiece or overlay to a shockpressure and residual stress profiles imparted to the workpiece, one candetermine the effectiveness or success of the laser processing of theworkpiece. Therefore, it is no longer necessary to use a destructivetechnique, such as x-ray diffraction, to evaluate the impartedcompressive residual stresses during laser shock processing.

[0019] Another advantage of the present invention is the ability to doreal time quality control of a workpiece subjected to laser shockprocessing. Since measured characteristics of acoustic and spectralemissions may be correlated to shock pressure and compressive residualstresses imparted in a workpiece, one can perform quality control inreal time of a laser shock processed workpiece. By measuring acoustic orspectral emissions, one can determine the imparted compressive residualstress of a workpiece. If the correlated compressive residual stress isnot within a predetermined or desired range, additional laser shockprocessing may be done to ensure the desired compressive residual stressin the workpiece is achieved.

[0020] Prior to this invention, there was no known real time techniqueavailable for determining the compressive residual stresses imparted ina specific workpiece to provide quality control of that specificworkpiece. Prior to this invention, a technique of random or regulartesting was done on a workpiece subsequent to laser shock peening a lotof workpieces. The randomly selected workpiece was subjected to thedestructive technique of x-ray diffraction in which the impartedcompressive residual stress was measured in the randomly selectedworkpiece. The results of the randomly selected workpiece's residualstress profile was then extrapolated to non-tested workpieces processedunder the same condition, to estimate the non-tested workpieces'imparted compressive residual stress profile.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The above-mentioned and other features and advantages of thisinvention, and the manner of attaining them, will become more apparentand the invention will be better understood by reference to thefollowing description of an embodiment of the invention taken inconjunction with the accompanying drawings, wherein:

[0022]FIG. 1 is a cross sectional, diagrammatic view of an apparatus formonitoring laser shock processing of a workpiece according to thepresent invention.

[0023] Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplification set out hereinillustrates one preferred embodiment of the invention, in one form, andsuch exemplification is not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The improvements in fatigue life produced by laser shockprocessing are the result of residual compressive stresses developed inthe irradiated surface retarding fatigue crack initiations and/orslowing the crack propagation rate. A crack front is the leading edge ofa crack as it propagates through the solid material. Changes in theshape of a crack front and slowing of the crack growth rate when thecrack front encounters the laser shocked zone in a laser shockprocessing condition have been shown. Laser shock processing is aneffective method of increasing fatigue life in metal workpieces bytreating fatigue critical regions.

[0025] For a more thorough background in the prior history of lasershock processing and that of high power processing of engineeredmaterials, reference can be made to U.S. Pat. No. 5,131,957. Such patentis hereby incorporated by reference. This patent also shows a type oflaser and laser configuration adaptable for use with the presentinvention. Another type of laser adaptable for use with the presentinvention is that of a Nd-Glass laser manufactured by LSP Technologies,Inc. of Dublin, Ohio.

[0026] Overlays are applied to the surface of the target workpiece beinglaser shock processed. These overlay materials may be of two types, onetransparent to laser radiation and the other opaque to laser radiation.They may be used alone or in combination with each other, but it ispreferred that they be used in combination with an opaque layer adjacentto the workpiece and the outer transparent layer being adjacent to theopaque layer.

[0027] Referring now to FIG. 1, components of one embodiment of anapparatus for monitoring laser shock processing of a workpiece areshown. Opaque overlay 10 and transparent overlay 12 are applied toworkpiece 14. A beam of coherent energy or laser pulse 16 is directedfrom laser 18 through transparent overlay 12 and is absorbed by opaqueoverlay 10.

[0028] During laser shock processing, the laser pulse 16 is absorbed byopaque layer 10, which is quickly vaporized, producing a plasma. Theplasma is confined by the transparent overlay 12 resulting in a pressurepulse applied to workpiece 14. The pressure pulse creates a shock wavewithin workpiece 14, which imparts deep compressive stresses withinworkpiece 14.

[0029] When opaque layer 10 is hit with a laser pulse 16, acousticenergy 20 and spectral emission 22 are emitted from workpiece 14 andopaque overlay 10. Radiometer 24 detects spectral emissions and acousticdetector 26 detects acoustic energy emissions.

[0030] Radiometer 24 measures a portion of the optical spectrum and isdirected toward workpiece 14, and in particular, that of opaque overlay10 such that radiometer 24 may measure the temperature of the workpieceand/or the plasma when they are irradiated by a laser pulse 16.

[0031] In addition to radiometer 24, the temperature of workpiece 14 maybe measured by other means. For example, a thermocouple or simple IRthermometer (not shown) may be used to measure the rear surfacetemperature of the workpiece separately. Traditionally, an IRthermometer is a type of radiometer normally used to measure much lowertemperatures than that of the plasma.

[0032] During laser shock processing, spectral emissions 27 are emittedfrom various components involved in laser shock processing. The spectralemissions 22 are the result of workpiece 14 and/or opaque layer 10 beingsubjected to a laser pulse 16. When workpiece 14 is vaporized to producea plasma, spectral emissions 22 are emitted. In addition, whentransparent overlay 12, in contact with opaque overlay 10 or workpiece14 is vaporized to produce a plasma, spectral emissions 22 are emitted.

[0033] Opaque overlay 10 may include a dopant. The dopant material mayresult in a detectable optical or spectral emission. The opaque orenergy absorbing layer 10 may also fluoresce when the opaque layer 10 issubjected to pressure or temperature.

[0034] Alternatively or in addition to a dopant in opaque overlay,transparent overlay 12 may include a dopant. The dopant material mayresult in a detectable optical or spectral emission. The opaque orenergy absorbing layer 10 may also fluoresce when the opaque layer 10 issubjected to pressure or temperature.

[0035] The dopant may be dispersed in particle form, solute form, or anyrecognized dispersant form in the opaque overlay to a transparentoverlay 12. The dopant may also consist of micro-spheres filled with amarker. When the micro-spheres are subjected to temperature or pressure,the micro-spheres are crushed or fractured, releasing their marker. Thereleased marker is then detected by radiometer 24 or other detectiondevices.

[0036] During operation of the present invention, when a beam ofcoherent energy or laser pulse 16 is absorbed by opaque layer 10,acoustic energy 20 is emitted from workpiece 14, resulting from theeffects of the shockwave and from the rapidly expanding plasma. Althoughnot depicted in FIG. 1, a plurality of radiometers may be utilized. Forexample, multiple radiometers, each one observing a different band suchas UV, visible, and IR, or narrower bands may be utilized.

[0037] For real-time or in-process monitoring of the effectiveness oflaser shock processing of workpieces, means and methods for measuringthe characterizing parameters of temperature and pressure in the plasmaformed in front of the material by the laser pulse are needed. Thisplasma is the source of the shockwave, which produces a beneficialmodification of the material as it traverses the workpiece. Theseparameters (temperature and pressure) are not independent of one anotherbut are related by a complex equation of state. The parameters followtemporal histories depending upon this equation of state, thetransparent overlay material composition and thickness, the opaqueoverlay material composition and thickness, the substrate compliance,the laser beam irradiance and temporal history, and the laserwavelength.

[0038] It is not necessary to have a complete predictive model for theplasma in order to use these parameters to monitor the laser shockprocessing conditions. By performing measurements of plasma temperatureand pressure (both magnitude and temporal history, i.e., signatures)under laser and material conditions for which known processing resultsare obtained, temperature and pressure sensors can be calibrated tothese processing results for use as real time or in- process qualitycontrol monitors. The temperature of the plasma may be able to providean indirect measure of the plasma pressure, which is directly related tothe shock intensity in the material (and hence, laser shock processingeffectiveness). Temperature measurements can also detect the absence ofthe transparent overlay, the absence of the opaque overlay, or both.

[0039] An indirect pressure measurement may be utilized in the presentinvention. A simple model for the evolution of the plasma formed inlaser shock processing is the piston model. In this model, a fixedamount of the opaque overlay is heated adiabatically by absorption ofthe laser pulse. The model predicts a power law dependence of peakpressure Po on peak irradiance, G, Po=G^(½), for fixed irradiancetemporal profile, and a fixed fraction of incident energy given toionization. Although the model oversimplifies the physics, the basicdependence of pressure on irradiance has been verified experimentally.Under these same assumptions, the temperature should follow the samepower law dependence on irradiance. Temperature in the confined plasma(confined by transparent overlay) has been measured to be about 8,000 Kat 1 GW/cm², but the irradiance dependence has not been confirmed. Fornormal laser shock processing conditions, the temperature should be inthe range of 15,000 K (at 3.5 GW/cm²) to 25,000 K (at 10 GW/cm²) andpressure should be close to a linear relationship with a measuredtemperature.

[0040] The measurement of plasma temperature is relatively uncomplicatedfor plasmas that emit radiation as a blackbody in portions of thespectrum. If a radiometer is arranged to view the plasma with filtersadmitting plasma light in a narrow spectral range where the plasma isradiating as a blackbody, then the radiometer output signal can bedirectly related to plasma temperature by the Planck function. Such aradiometer can be constructed with simple lenses, optical filters, and asilicon PIN photodiode for the UV/VIS/NIR portions of the spectrum.

[0041] Alternate means of measuring the plasma temperature rely onviewing the plasma in portions of the spectrum where individual spectrallines of atomic or ionic species in the plasma are visible. The atomicor ionic species might be the abundant species of the overlay materials,such as C, H, O or might be trace elements introduced as dopants for thepurpose of monitoring temperature. Trace elements should normally bebenign low atomic number elements such as B, Li, N, Na, Mg, Al, Si, butcould be any element with co-located strong lines arising from differentenergy levels. Because pumping rates of the upper levels are stronglytemperature dependent, ratios of line emissions provide a sensitivemeans of measuring plasma temperature. The line ratio approach may beparticularly useful for laser shock processing because the plasmatemperature is a weak function of irradiance and a sensitive measurementof temperature is desirable. Implementation of the line ratiomeasurement can be achieved with three fast photodiodes and threenarrow-band filters, two centered on the target emission lines and oneon the continuum in between.

[0042] The present invention can be utilized to detect the absence of anoverlay. In the case where the transparent overlay is missing, theplasma is unconfined and quickly expands to low density. The effect ofthe expansion is to put more energy into fewer atoms than in theconfined plasma case. This leads to higher plasma temperatures. Priorresearch has shown that the temperature dependence on fluence forexpansion into a vacuum follows a power law, F^(½), where F is incidentbeam fluence. Typical temperatures for a black paint overlay with noconfinement are 50,000 K at 200/J/cm² (8 Gw/cm²) and 110,000 K at1000/J/cm² (40 Gw/cm²) for a 25-ns pulse. These temperature levels aregreater than those predicted for the confined case at similar fluences.Temperatures will be even higher if there is time to establish alaser-supported detonation wave in the air in front of the expandingplasma, which is the condition expected for a missing transparentoverlay. A thin overlay might also be detected by an increase in plasmatemperature in the middle of the laser pulse when the reflected shockwave from the water-free surface arrives at the water-plasma surface.Plasma temperature measurements may by implemented by the methodsmentioned above.

[0043] An alternative method for determining the absence of thetransparent overlay, is to dope the overlay with a small amount offluorescent material. Under normal operation, the intense UV from theplasma will cause the overlay to fluoresce at characteristicwavelengths. The fluorescent emissions would be missing or weak for amissing or thin overlay. The fluorescent materials or viewing geometriesmust be selected such that the emissions dominate the background plasmaradiation. This might be accomplished with dyes or dopant materialshaving fluorescent lifetimes long compared to the plasma lifetime. Insuch cases, fluorescing droplets of overlay material could be detectedafter they are ejected from the surface and are distant from thebackground plasma.

[0044] A third alternative method for determining the absence of atransparent overlay is to sense the overlay droplet cloud byshadowgraphy or light scattering under normal operations. The cloudsignatures would be missing or altered if the overlay were missing orthin.

[0045] The present invention may also be used to detect the absence ofthe opaque overlay. In the event that the opaque overlay is missing, thetemperature signature would be different than normal, because, ingeneral, the equation of state of the substrate material will be quitedifferent from that of the opaque overlay (e.g., iron versus carbon). Amore sensitive detector, however, will be a fast photodiode with a linefilter centered on a known strong emission line of a major constituentof the substrate. Generally, substrate species can be found thattypically are not present in either the transparent overlay or theopaque overlay.

[0046] The strength of a shockwave imparted to the workpiece is ideallydetermined by a direct measurement of pressure in the plasma. Anindirect measurement of pressure may be done using plasma emissions andspectroscopic techniques. Stark broadening of spectral lines occurs withincreasing electron density due to the associated increase in averagelocal electric field. This mechanism has been used to measure electrondensity in plasmas containing oxygen. The peak of the oxygen emissionline also exhibits a red shift with increasing electron density. Theelectron density multiplied by temperature contributes to the totalplasma pressure in a predictable manner. Thus, spectral line broadeningand/or shifts in selected elemental emission lines, combined withtemperature measurements, should provide a measure of the plasmapressure. These measurements may be implemented with a spectrometer oran array of fast photodiodes with narrow-band line filters selected toencompass the line of interest. The atomic specie used to serve as apressure indicator may be any specie naturally occurring in the opaqueor transparent overlay material such as C, H, or O, or it may be addedto the overlay materials as a trace element specifically for the purposeindicating pressure, e.g., B, Li, N, Na, Mg, Al, or Si.

[0047] The spectroscopic measurements of pressure discussed previouslyrequire careful emission measurements of the plasma radiation inspectral regions where the radiation is not dominated by the continuum.The benefit of this approach is the advantage of real time in-processmonitoring of the plasma pressures generated.

[0048] Once the temperature and/or pressure history in the plasma havebeen measured, the measured record may be correlated to the deepcompressive residual stresses imparted in the workpiece. The correlationinvolves laser shock processing a workpiece to impart compressiveresidual stresses while measuring temperature and/or pressure historiesof the plasma created. A residual stress profile of the workpiece isthen measured using the x-ray diffraction technique. A correlation orlook-up table is created which maps plasma pressure and temperaturehistories to the x-ray diffraction-measured residual stress profile.Once this correlation table has been created, one can look up thepredicted residual stress profile from the empirical data of temperatureand pressure temporal histories.

[0049] A residual stress profile may be determined by using x-raydiffraction techniques commonly known in the art. For example, an x-raybeam may be directed to a metal surface where the measurement is desiredand the x-ray beam is diffracted from the surface at an angle related tothe spacing of the atomic planes diffracting the beams. If the spacingbetween these planes changes, the angle of the diffraction peak shiftsslightly. The inter-planar distance will increase or decrease if thelocal elastic strains in the lattice are tensile or compressive,respectively. By measuring the shift in the diffraction peak, theelastic strain in the lattice can be determined and through the elasticmodulus, the residual stress causing this strain can be calculated. Toget the in-depth profile, a thin layer of the surface is removed byelectropolishing (grinding will create its own residual stresses in athin surface layer), and the measurement is repeated in the samelocation. This step-wise sequence is continued to the total depth ofmeasurement desired. A modeling program is then used to applycorrections to the measured values to account for the stress relaxationcaused by removing the successive layers of material.

[0050] The measured temperature and pressure history may also becorrelated to fatigue life in a similar manner in which empiricallymeasured temperature and pressure is correlated to a measured residualstress profile. Fatigue life may be measured by placing a workpiece in amachine that is exposed to a cyclic strain or stress. It is usuallydefined in terms of the number of cycles to failure under the testingconditions used. Fatigue life is defined as the maximum number of cyclesto failure at a given cyclic stress amplitude.

[0051] The method of correlating empirical temperature and pressure tofatigue life consists of laser shock processing while recordingtemperature and pressure of the produced plasma. Fatigue life of thelaser shock processed workpiece is then measured. Next, the measuredfatigue life is correlated to the measured temperature and pressure.These steps are repeated for varying combinations of measuredtemperatures and pressures to create a chart, graph, or correlationtable which correlates fatigue life to the measured temperatures andpressures. With this table, one can use empirically measured temperatureand pressure to determine a predicted fatigue life in a workpiece.

[0052] The measured acoustic signal or energy emitted during laser shockprocessing may also be used for determining a residual stress profile,fatigue life, or shock pressure applied to a workpiece. The methodincludes recording empirical data of acoustic signatures or acousticenergy emitted during different laser shock processing conditions.Subsequent to laser shock processing, a residual stress profile andfatigue life is determined, as described above. A correlation table,chart, or graph is then created which relates empirically recordedacoustic energy to workpiece characteristics, such as residual stressprofile or fatigue life. This correlation table, chart, or graph maythen be used in subsequent laser shock processing cycles for determininga predicted residual stress profile, fatigue life, or shock pressureapplied to a workpiece through empirically recorded emitted acousticenergy.

[0053] Quality control is achieved through use of the present invention.One is now able to determine a predicted residual stress profile orfatigue life of a workpiece from the observed laser processingconditions in real time. This allows one to determine the consistency oflaser shock processing conditions during production processing. If thepredicted residual stress profile, fatigue life, or pressure pulse isnot within a predetermined or desired range, one can now reprocess theworkpiece, as necessary. In addition, one can use this method forproducing uniform compressive residual stresses in a workpiece.

[0054] While this invention has been described as having a preferreddesign, the present invention can be further modified within the spiritand scope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principles. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains and which fallwithin the limits of the appended claims.

What is claimed is:
 1. An apparatus for monitoring laser shockprocessing of a workpiece, comprising: a material applicator forapplying an energy absorbing material onto the workpiece; a transparentoverlay applicator for applying a transparent overlay onto the workpieceover said energy absorbing layer; a laser operatively associated withsaid energy absorbing layer; and at least one radiometer.
 2. Theapparatus of claim 1 further comprising a second radiometer, saidradiometers detecting energy at different wavelengths.
 3. The apparatusof claim 1 wherein said energy absorbing material contains a dopant. 4.The apparatus of claim 3 wherein the presence of said dopant results ina detectable optical signal when said dopant is subjected to pressure.5. The apparatus of claim 3 wherein the presence of said dopant resultsin a detectable optical signal when said dopant is heated.
 6. Theapparatus of claim 1 wherein said energy absorbing material fluoresceswhen said energy absorbing material is subjected to heat.
 7. Theapparatus of claim 1 wherein said energy absorbing material fluoresceswhen said energy absorbing material is subjected to pressure.
 8. Theapparatus of claim 1 wherein said energy absorbing material emitsoptical energy detectable by said radiometer.
 9. The apparatus of claim3 wherein said dopant consists of micro-spheres filled with a marker.10. A method for real time laser shock processing of a workpiececomprising the steps of: applying an opaque overlay to the workpiece;directing a beam of coherent energy to the workpiece to vaporize aportion of the opaque overlay and create a plasma which emits energytherefrom; and monitoring a portion of the energy emitted from theplasma.
 11. The method for real time monitoring laser shock processingof a workpiece according to claim 10 wherein the step of monitoring aportion of the energy emitted from the plasma further comprises thesteps of: detecting spectral emissions from said emitted energy.
 12. Themethod for real time monitoring laser shock processing of a workpieceaccording to claim 10 wherein the step of monitoring a portion of theenergy emitted from the plasma further comprises the steps of: detectingacoustic emissions from said emitted energy.
 13. A method for real-timemonitoring the laser shock peening of a workpiece comprising the stepsof: applying a transparent overlay to the workpiece; directing a beam ofcoherent energy to a workpiece through said transparent overlay andcreate a plasma which emits energy therefrom; and monitoring a portionof the energy emitted from said plasma.
 14. The method of claim 13 ,wherein said step of monitoring a portion of energy further comprisesthe step of: detecting spectral emissions from said plasma emittedenergy.
 15. The method of claim 13 , wherein said step of monitoring theplasma further comprises the step of: detecting acoustic emissions fromsaid emitted energy.
 16. The method of claim 11 , wherein said step ofdetecting spectral emissions further comprises the step of: correlatinga feature of said spectral emissions to the shock pressure applied tothe workpiece.
 17. The method of claim 12 , wherein said step ofdetecting acoustic emissions further comprises the step of: correlatinga feature of said acoustic emissions to the shock pressure applied tothe workpiece.
 18. The method of claim 14 , wherein said step ofdetecting spectral emissions further comprises the step of: correlatinga feature of said spectral emissions to the shock pressure applied tothe workpiece.
 19. The method of claim 15 , wherein said step ofdetecting the acoustic emissions further comprises the step of:correlating a feature of said acoustic emissions to the shock pressureapplied to the workpiece.
 20. The method of claim 11 , further comprisesthe step of correlating a feature of said detected spectral emissions toa residual stress profile produced in the workpiece.
 21. The method ofclaim 12 , wherein said step of detecting acoustic emissions furthercomprises the step of: correlating a feature of said acoustic emissionsto a residual stress profile produced in the workpiece.
 22. The methodof claim 14 , wherein said step of detecting spectral emissions furthercomprises the step of: correlating a feature of said spectral emissionsto a residual stress profile produced in the workpiece.
 23. The methodof claim 15 , wherein said step of detecting acoustic emissions furthercomprises the step of: correlating a feature of said acoustic emissionsto a residual stress profile produced in the workpiece.
 24. The methodof claim 11 , wherein said step of detecting spectral emissions furthercomprises the step of: correlating a feature of said spectral emissionsto the fatigue life of the workpiece.
 25. The method of claim 12 ,wherein said step of detecting acoustic emissions further comprises thestep of: correlating a feature of said acoustic emissions to the fatiguelife of the workpiece.
 26. The method of claim 14 , wherein said step ofdetecting the spectral emissions further comprises the step of:correlating a feature of said spectral emissions to the fatigue life ofthe workpiece.
 27. The method of claim 15 , wherein said step ofdetecting acoustic emissions further comprises the step of: correlatinga feature of said acoustic emissions to the fatigue life of theworkpiece.
 28. The method of claim 11 , wherein said step of detectingspectral emissions further comprises the step of: correlating a featureof said spectral emissions to the presence of a transparent overlay onthe workpiece.
 29. The method of claim 12 , wherein said step ofdetecting acoustic emissions further comprises the step of: correlatinga feature of said acoustic emissions to the presence of a transparentoverlay on the workpiece.
 30. The method of claim 11 , wherein said stepof detecting spectral emissions further comprises the step of:correlating a feature of said spectral emissions to the presence of anopaque overlay on the workpiece.
 31. The method of claim 12 , whereinsaid step of detecting acoustic emissions further comprises the step of:correlating a feature of said acoustic emissions to an opaque overlay onthe workpiece.
 32. A method for real-time monitoring the laser shockpeening of a workpiece comprising the steps of: applying an opaqueoverlay to a workpiece; directing a beam of coherent energy to aworkpiece to vaporize a portion of said opaque overlay and create aplasma thereon; and monitoring the temperature of said plasma.
 33. Themethod of claim 32 , further comprises the step of creating a shockpressure on the workpiece: and correlating the plasma temperature to theshock pressure on the workpiece.
 34. The method of claim 32 , furthercomprising the steps of creating a residual stress profile in theworkpiece; and correlating the plasma temperature to the residual stressprofile provided in the workpiece.
 35. The method of claim 32 , whereinsaid step of monitoring the plasma temperature further comprises thestep of: correlating the plasma temperature to the fatigue life of theworkpiece.
 36. A method for real-time monitoring the laser shockprocessing of a workpiece comprising the steps of; applying atransparent overlay to a workpiece; directing a beam of coherent energyto a workpiece through said transparent overlay and create a plasmathereon; and monitoring the temperature of said plasma.
 37. The methodof claim 36 , wherein said step of monitoring the plasma temperaturefurther comprises the step of: correlating the plasma temperature to theshock pressure on the workpiece.
 38. The method of claim 36 , whereinsaid step of monitoring the plasma temperature further comprises thestep of: correlating the plasma temperature to the residual stressprofile provided in the workpiece.
 39. The method of claim 36 , whereinsaid step of monitoring the plasma temperature further comprises thestep of: correlating the plasma temperature to the fatigue of life ofthe workpiece.
 40. The apparatus of claim 6 , further comprising: alaser pulse of coherent energy; and said laser pulse produces said heat.41. The method of claim 14 , wherein said step of detecting spectralemissions further comprises the step of: correlating a feature of saidspectral emissions to the presence of a transparent overlay on theworkpiece.
 42. The method of claim 15 , wherein said step of detectingacoustic emissions further comprises the step of: correlating a featureof said acoustic emissions to the presence of a transparent overlay onthe workpiece.
 43. The method of claim 14 , wherein said step ofdetecting spectral emissions further comprises the step of: correlatinga feature of said spectral emissions to the presence of an opaqueoverlay on the workpiece.
 44. The method of claim 15 , wherein said stepof detecting acoustic emissions further comprises the step of:correlating a feature of said acoustic emissions to an opaque overlay onthe workpiece.